Compare circuitry with scan cell separate from serial scan circuitry

ABSTRACT

A scan test architecture facilitates low power testing of semiconductor circuits by selectively dividing the serial scan paths into shorter sections. Multiplexers between the sections control connecting the sections into longer or shorted paths. Select and enable signals control the operation of the scan path sections. The output of each scan path passes through a multiplexer to compare circuits on the semiconductor substrate. The compare circuits also receive expected data and mask data. The compare circuits provide a fail flag output from the semiconductor substrate.

RELATED PATENTS/APPLICATIONS

This application is a divisional of prior application Ser. No.13/230,367, filed Sep. 12, 2011, now U.S. Pat. No. 8,190,954, grantedMay 29, 2012;

-   Which was a divisional of prior application Ser. No. 12/966,127,    filed Dec. 13, 2010, now U.S. Pat. No. 8,037,383, granted Oct. 11,    2011;-   Which was a divisional of prior application Ser. No. 12/716,853,    filed Mar. 3, 2010, now U.S. Pat. No. 7,877,650, granted Jan. 25,    2011;-   Which was a divisional of prior application Ser. No. 12/033,163,    filed Feb. 19, 2008, now abandoned;-   Which was a divisional of prior application Ser. No. 11/051,708,    filed Feb. 4, 2005, now U.S. Pat. No. 7,356,745, granted Apr. 8,    2008;-   Which claims priority from Provisional Application No. 60/542,410,    filed Feb. 6, 2004.

This application is related to the following US patents/applicationswhich are incorporated herein by reference.

-   Application Ser. No. 09/803,599, filed Mar. 9, 2001, now U.S. Pat.    No. 6,769,080, issued Jul. 27, 2004.-   Application Ser. No. 09/896,467, filed Jun. 29, 2001, now U.S. Pat.    No. 6,717,429, issued Apr. 6, 2004.-   Application Ser. No. 09/997,462, filed Nov. 29, 2001, now U.S. Pat.    No. 6,766,487, issued Jul. 20, 2004.-   Application Ser. No. 10/301,898, filed Nov. 22, 2002, now U.S. Pat.    No. 6,894,308, issued May 17, 2005.-   Application No. 60/542,810, filed Feb. 6, 2004, now application Ser.    No. 11/051,696, filed Feb. 4, 2005, abandoned.

BACKGROUND OF THE DISCLOSURE Technical Field of the Disclosure

This disclosure relates in general to integrated circuit design andtesting, and in particular to a circuit test architecture that enableslow power testing of embedded circuit core functions using a simplifiedtester interface.

A System On a Chip (SOC) design may consist of many types of embeddedcore functions such as DSPs, CPUs, memories, and various other types.Typically the testing of embedded cores is achieved by scan testing thecores. Conventional core scan testing is achieved by placing the core ina test mode whereby scan inputs to the core and scan outputs from thecore are made available for access by a tester external to the SOC.

While there are many publications on scan testing, a paper by Marinissen(Scan Chain Design for Test Time Reduction in Core Based ICs”, 1998 IEEEInternational Test Conference) is chosen to provide backgroundinformation on conventional methods of scan testing cores in an SOC.This paper describes the following three types of core scan testconfigurations that can be used in an SOC.

The first core scan test configuration described in regard to FIG. 1 ofthe paper is referred to as a multiplexing architecture. In themultiplexing architecture, the scan inputs of multiple cores areconnected to a common scan input bus from a tester while the scanoutputs of the multiple cores are individually multiplexed to a commonscan out bus to the tester. During test, the tester individually selectsone core at a time and tests the core using the scan input and scanoutput bus. The test is complete after all cores have been individuallyselected and tested.

The second core scan test configuration described in regard to FIG. 2 ofthe paper is referred to as a daisychain architecture. In the daisychainarchitecture, the scan inputs and outputs of multiple individual coresare serially connected to form a daisychained scan path from thetester's scan input to the tester's scan output. During test, the testeraccesses the daisychained core scan paths and applies the scan test tothe cores. The test is complete after all cores on the daisychain scanpath have been tested.

The third core scan test configuration described in regard to FIG. 3 ofthe paper is referred to as a distribution architecture. In thedistribution architecture, the scan inputs and outputs of each core aremade individually accessible by a tester. This means that the testerneeds to have a scan input and scan output bus dedicated for testingeach core. During test, the tester accesses each core's scan input andscan output buses and tests each core in parallel. The test is completeafter all cores in the distribution architecture have been tested.

From the above description it is clear that each of the scan testconfigurations require the tester to receive, either directly as in themultiplexing and distribution architectures or indirectly as in thedaisychain architecture, the scan outputs from each core being tested.Having to include circuitry in testers for receiving scan output datafrom an SOC increases the cost of the tester and leads to larger testaccess interfaces between the tester and SOC. It should also be clearthat the power consumed and heat generated during parallel core testingusing the distribution and daisychain configurations may limit thenumber of cores that may be tested in parallel and force the testing tooccur sequentially over a number of smaller core groups. Having topartition a plurality of cores to be tested into small separately testedgroups to manage SOC power consumption and heat generation leads tolonger test times and increases the cost of the SOC.

SUMMARY OF THE DISCLOSURE

The present disclosure provides a method and apparatus for testing coresin an SOC using lower cost testers that operate to test SOCs using areduced test access interface between the SOC and tester. In addition,the present disclosure provides a method and apparatus for increasingthe number of cores that can be tested in parallel by providing a corecircuit test architecture that lowers the power consumption and heatgeneration during test.

Scan testing of an integrated circuit generally occurs by shiftingstimulus data into a serial scan path from a tester, applying thestimulus data to functional circuits, capturing response data in thescan path, and shifting the captured response data out to a tester.

The disclosed scan test architecture facilitates low power testing ofsemiconductor circuits by selectively dividing the serial scan pathsinto shorter sections. Multiplexers between the sections controlconnecting the sections into longer or shorted paths. Select and enablesignals control the operation of the scan path sections. The output ofeach scan path passes through a multiplexer to compare circuits on thesemiconductor substrate. The compare circuits also receive expected dataand mask data. The compare circuits provide a fail flag output from thesemiconductor substrate.

This arrangement selectively provides a standard power mode of operationand a low power mode of operation. In the standard power mode ofoperation all the scan path sections are shifted at the same time. Inthe low power mode of operation only one section or less than all thesections are shifted at one time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a first low power core circuit test architectureaccording to the disclosure.

FIG. 1A illustrates the compare circuit of FIG. 1.

FIG. 1B illustrates a circuit of FIG. 1A.

FIG. 2 illustrates a first low power scan path arrangement according tothe present disclosure.

FIG. 3 illustrates a test control sequence for the low power scan patharrangement of FIG. 2 according to the present disclosure.

FIG. 4 illustrates a first example of using the FIG. 1 circuit testarchitecture in an SOC according to the present disclosure.

FIG. 5 illustrates second example of using the FIG. 1 circuit testarchitecture in an SOC according to the present disclosure.

FIG. 6 illustrates third example of using the FIG. 1 circuit testarchitecture in an SOC according to the present disclosure.

FIG. 7 illustrates a second low power scan path arrangement according tothe present disclosure.

FIG. 8 illustrates a test control sequence for the low power scan patharrangement of FIG. 7 according to the present disclosure.

FIG. 9 illustrates a second low power core circuit test architectureaccording to the disclosure.

FIG. 10 illustrates a first example of using the FIG. 9 circuit testarchitecture in an SOC according to the present disclosure.

FIG. 11 illustrates second example of using the FIG. 9 circuit testarchitecture in an SOC according to the present disclosure.

FIG. 12 illustrates third example of using the FIG. 9 circuit testarchitecture in an SOC according to the present disclosure.

FIG. 13 illustrates an example of testing a plurality of SOC die onwafer using the low power core circuit test architectures of FIGS. 4 and10 according to the present disclosure.

FIG. 14 illustrates an example of testing a plurality of SOC die onwafer using the low power core circuit test architectures of FIGS. 5 and11 according to the present disclosure.

FIG. 15 illustrates an example of testing a plurality of SOC die onwafer using the low power core circuit test architectures of FIGS. 6 and12 according to the present disclosure.

FIG. 16 illustrates an example of testing a plurality of packaged ICs ona multiple IC test fixture using the low power core circuit testarchitectures of FIGS. 4 and 10 according to the present disclosure.

FIG. 17 illustrates an example of testing a plurality of packaged ICs ona multiple IC test fixture using the low power core circuit testarchitectures of FIGS. 5 and 11 according to the present disclosure.

FIG. 18 illustrates an example of testing a plurality of packaged ICs ona multiple IC test fixture using the low power core circuit testarchitectures of FIGS. 6 and 12 according to the present disclosure.

FIG. 19 illustrates an example of testing a plurality of FIG. 13 waferson a multiple wafer test fixture according to the present disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

FIG. 1 illustrates a simplified example of a low power core circuit testarchitecture 100 that enables cores within an SOC to be scan tested inparallel. The functional core circuitry has been placed into a scan testconfiguration whereby the functional flip flops of the core areconverted into low power parallel scan paths 104 that are used tocommunicate test patterns to and from combinational logic 102 of thecore via connections 110.

The test architecture 100 includes compare circuitry 106 at the outputs128 of the parallel scan paths 104 to allow the test response data 128from each parallel scan path to be compared with expected data 112 inputfrom a tester. The compare circuitry 106 also contains masking circuitryto allow for mask data 114 input from the tester to selectively mask offcertain comparisons between expected data 112 and response data 128 toavoid comparing against unknown response outputs. The compare circuitry106 is dedicated for use in testing the core of test architecture 100and accompanies the core when the core is used within an SOC. Theexpected data input 112, mask data input 114, stimulus data input 116,control input 118, and enable input 120 to circuit 100 are shown to forman test input bus 126 from a tester.

During test, a tester inputs stimulus data 116 to the inputs of theparallel scan paths 104, control 130 from control bus 118 to the controlinputs of the scan path 104, and an enable input 120 to the enable inputof the scan path. When enabled by enable input 120, the control inputsoperate the parallel scan paths in a low power scan mode to shift in andapply stimulus data to combinational logic 102, capture the responsedata output from combinational logic 102 to the applied stimulus, and toshift out the captured response data to compare circuitry 106. When notenabled by enable input 120, the control inputs still perform the sameshift in, capture, and shift out operations but in a conventional scanmode, not the low power scan mode. This will be described later inregard to FIG. 2. Control 132 from control bus 118 is input to thecompare circuitry 106 to operate the compare circuitry during test.During test, the compare circuitry 106 matches the response 128 outputfrom the parallel scan paths 104 with expected data 112 input from thetester, unless the match operation is disabled by mask data input 114from the tester. A signal occurs on the fail flag output 124 whenever amismatch occurs between the expected and response data to notify thetester of the failure.

In FIGS. 1A and 1B, the compare circuitry 106 contains a compare andmask circuit element 134 for each individual parallel scan path outputon bus 128. An XNOR gate 136 is provided in the compare and mask element134 to serve as a comparator between an expected data input 112 from thetester and a response data output 128 from a parallel scan path. An ORgate 138 is provided to mask off the compare result from the XNOR 136when controlled to do so by mask data input 114 from the tester. A faillatch (FL) 140 is provided to store all compare results output from XNOR136 that are not masked by OR gate 138. The fail latch 140 is strobed bycontrol from control bus 132 each time response data 128 from theparallel scan paths is compared with expected data 112 from the tester.

A scan cell (SC) 142 is provided to allow capturing the value in thefail latch 140 and shift it out following a test. The scan cell 142comprises a typical flip flop and multiplexer combination enabling thecapturing and shift of data. The scan cell 142 is operated by controlfrom control bus 132. A non-inverting open drain buffer 144 is providedto allow a fail flag output 150 from each compare and mask element 134.The fail flag output 150 serves as an immediate indicator to the testerthat a mismatch between expected and response data has occurred and isstored in the fail latch 140.

The open drain buffer 144 is used to allow wire-ORing all fail flagoutputs 150 from all compare and mask elements 134 together to produce awired-OR fail flag output 124 to the tester from compare circuitry 106.A pull up element 108 exists on the wired-OR fail flag 124 to pull theoutput high. If a failure occurs on any one of the individual fail flagoutputs 150, the wired-OR fail flag 124 will be pulled low. The failflag pull up element 108 may be located internal to or external ofarchitecture 100.

The scan inputs 146 and scan outputs 148 of all compare and maskelements 134 within compare circuitry 106 are serially connected to formthe scan path between the scan input 121 and scan output 122 of comparecircuitry 106. The tester can access the compare circuitry 106 scan pathfollowing a test operation to shift out the pass/or fail values storedin fail latches 140.

In the compare and mask element 134 example, if a mismatch betweenexpected 112 and response 128 data input to XNOR gate 136 occurs, thefail latch 140 will be set to a failure indicating logic zero statewhich will persist until the end of test. If no mismatches occur duringtest, the XNOR will output logic ones and the fail latch will remain inthe pass indicating logic one state. The fail flag is initialized at thebeginning of a test to a logic one by control from control bus 134. Alogic one mask input 114 from the tester will force a logic one from ORgate 138 to the fail flag 140 independent of the output of XNOR 136 toachieve the compare masking feature.

The use of compare circuitry in an integrated circuit for the purpose oftesting embedded circuits and cores similar to that described above wasintroduced by referenced U.S. Pat. No. 6,560,734. The use of compare andmask circuitry in an integrated circuit for the purpose of testingembedded circuits and cores similar to that described above wasintroduced in referenced U.S. patent application Ser. Nos. 09/896,467;10/301,898; and 60/542,810.

FIG. 2 illustrates in more detail the low power scan path 104 of FIG. 1.The low power scan path is constructed by partitioning the overalllength of a conventional scan path 200 into multiple separate parallelscan path sections, each section being of equal or near equal length. Inthis example, three separate sections A 202, B 204, and C 206 have beenformed from the original parallel scan path 200. Section A 202 receivesthe stimulus input bus 116 and outputs to multiplexers 208 and 212.Section B 204 selectively receives input from either stimulus bus 116 orfrom the scan outputs of section A 202, via multiplexer 208, and outputsto multiplexers 210 and 212. Section C 206 selectively receives inputfrom either the stimulus bus 116 or from the scan outputs from section B204, via multiplexer 210, and outputs to multiplexer 212.

Section A 202 receives scan control input 234 from gating circuitry 214,section B 204 receives scan control input 236 from gating circuitry 216,and section C 206 receives scan control 238 from gating circuitry 218.Gating circuitry 214, 216, and 218 each receive common scan controlinput from control bus 132 and each receive separate gating enableinputs 222, 224, and 226 from select controller 220. The selectcontroller 220 also outputs multiplexer controls to multiplexers 208,210, and 212 on respective leads 228, 230 and 232. Select controller 220inputs control from control bus 130 and the enable input 120.

During scan test operations when the enable input 120 is set to enablethe low power mode of low power scan path 104, each parallel scan pathsection A, B, and C is accessed separately by controller 220 such thatonly portions of the overall stimulus data signals on bus 110 tocombinational logic 102 change at a time. This method of scanning lowersthe power consumption in the combinational logic 102 since only portionsof the combinational logic inputs transition at a given time. Also thismethod of scanning lowers clocking power consumption by only allowingthe clocks on buses 234-238 to the currently selected section (i.e. A,B, or C) to operate at a give time.

During scan test operations when the enable input 120 is not set toenable the low power mode of low power scan path 104, each parallel scanpath section A, B, and C is connected in series together betweenstimulus bus 116 and response bus 128, via multiplexers 208, 210, and212, such that all sections are accessed simultaneously during scan testoperations. This mode of operation duplicates the operation of theconventional parallel scan path 200, and, in doing so, does not providethe low power mode of operation described above. To achieve this non-lowpower mode of operation, the controller 220 is designed such that whenthe enable signal 120 is not set for low power mode, the enable inputs222-226 to gating circuitry 214-218 are set to allow all the scan pathsections A 202, B 204, and C 206 to receive scan control from bus 130.Also, the inputs to multiplexers 208-212 are all set to concatenate thescan paths sections 202-206 together between stimulus bus 116 andresponse bus 128.

One reason for selectively providing both a low power and non-low powermode using the enable signal 120 is to provide a method of comparing orbenchmarking the actual power consumed during the low power mode ofoperation against the power consumed during the non-low power mode ofoperation. Another is that for some SOC designs with only a small numberof cores being tested in parallel, power consumption may not be aproblem and it may be desirable to simply operate the low power scanpath 104 as a conventional scan path 200. Still another reason is thatduring burn-in it may be desirable to operate the low power scan path104 in its conventional non-low power mode to actually use scanoperations to heat up the SOC during burn-in testing, eliminating theneed and cost of burn-in heaters/ovens.

FIG. 3 illustrates one preferred mode of operation of select controller220, when it is enabled, via enable input 120, for the low power mode ofoperation of scan path 104. The operation is depicted as a state diagramfor controller 220. At the beginning of each scan cycle, the controllerenters a state 302 to shift section A 202. In state 302, the stimulusbus 116 inputs to section A, the response bus 228 outputs from section A202 via multiplexer 212, and section A gating 214 is enabled to passscan control 130 to section A. When the shifting of section A iscomplete, the controller enters state 304 to shift section B 204.

In state 304, the stimulus bus 116 inputs to section B 204 viamultiplexer 208, the response bus 228 outputs from section B viamultiplexer 212, and section B gating 216 is enabled to pass scancontrol 130 to section B. When the shifting of section B is complete,the controller enters state 306 to shift section C 206. In state 306,the stimulus bus 116 inputs to section C 206 via multiplexer 210, theresponse bus 228 outputs from section C via multiplexer 212, and sectionC gating 218 is enabled to pass scan control 130 to section C.

When the shifting of section C is complete, the controller enters state308 to enable all gating circuits 214-218 to allow all scan pathsections A, B, and C to receive control from control bus 130 to capturedata from combinational logic 102. From state 308 the controller entersstate 302 to repeat the above described low power scan control stepsuntil the test is complete. The controller 220 shift states 302-306 andcapture state 308 transitions occur such that the test time of the lowpower scan path 104 is the same as the conventional non-low power scanpath 200.

With the exception of providing selective low power and non-low powermodes of operation, the low power scan path structure and operation asdescribed in FIGS. 2 and 3 above is similar to the structure andoperation of a low power scan path described in US Patent ApplicationPublication US 2001/0047498 A1, which is incorporated herein byreference.

FIG. 4 illustrates an example whereby a plurality of identical cores406-412 are embedded within the substrate 401 of a System On a Chip(SOC) integrated circuit 402. Each core includes an identical circuittest architecture 100. Since all the cores 406-412, their testarchitectures 100, and their test pattern set are identical, a tester404 may test all the cores in parallel. To achieve this parallel test,the SOC has been designed such that when it is placed in test mode apath is formed on the SOC to allow each test input bus 126 of eachcircuit test architecture 100 to be coupled to a test input bus 418 fromthe tester 404. Test input bus 418 connects to test input busses 126through terminals or bond pads 430 on substrate 401.

Also the serial inputs 121 and serial outputs 122 of each circuit testarchitecture 100 are serially connected together to form a scan pathbetween a scan input 414 at terminal 415 on substrate 401 from tester404 and a scan output 416 at terminal 417 to tester 404. Further, thefail flag outputs 124 from each circuit test architecture 100 arewire-ORed together and output at terminal 419 to a fail flag input 420to tester 404.

Terminals or bond pads 415, 417, 419, and 430 making electricalconnections to and from busses or signals on substrate 410 arespecifically depicted in FIG. 4. Like terminals or bond pads making likeelectrical connections on other semiconductor substrates are implied andto be understood, without being specifically shown for simplicity of thedrawings, in other structures depicted in the other drawing figures.

During test, the tester inputs common stimulus data 116, mask data 114,expected data 112, control 118, and enable 120 test input patterns toall circuits 100, and monitors the wired-OR fail flag input 420 from thecircuits 100. If a failure occurs during the test, as indicated by afail signal on fail flag input 420, the tester can either continue totest until the test is complete to catch other failures, or halt thetest on the occurrence of the first failure. If the test is a pass/failproduction test, the tester typically will cease testing on the firstoccurrence to save tester time.

However, if the test is a diagnostic test, the tester will typically notstop the test on a first failure but rather run the test to its normalcompletion to test for additional failures that may occur. In eithertest case, the tester 404 will typically follow up by scanning out thefail flag 140 of each circuit test architecture 100 after the test iscomplete, or aborted, to identify which one or more of the scan pathresponse outputs 128 failed during the test.

FIG. 5 illustrates an example whereby a first pair of identical cores506 and 508 and a second pair of identical cores 510 and 512 areembedded within an SOC 502. Identical cores 506 and 508 include anidentical circuit test architecture 100. Identical cores 510 and 512include an identical circuit test architecture 100. Since cores 506 and508, their test architectures 100, and their test pattern set areidentical, tester 504 may test both in parallel. Also, since cores 510and 512, their test architectures 100, and their test pattern set areidentical, tester 504 may test both in parallel.

Further, while the two pairs of cores are different both pairs may betested in parallel given that the tester 504 has separate test inputbuses 516 and 518. As seen in FIG. 5, tester test input bus 518 isdedicated to driving the test input buses 126 of the test architectures100 of cores 506 and 508, and tester test input bus 516 is dedicated todriving the test input buses 126 of test architectures 100 of cores 510and 512. As in the example of FIG. 4, the fail flag outputs 124 of thetest architectures 100 of cores 506-512 are wire-ORed for input totester 504 via fail flag input 420. Also, as with the example in FIG. 4,the scan inputs 121 and scan outputs 122 of the test architectures 100of cores 506-512 form a serial path between the tester's scan input 414and scan output 416.

The testing of each core pair occurs as described in regard to thetesting cores in FIG. 4. Since the core pairs are different from oneanother, one pair may complete its test before the other pair completesits test. If this happens, tester 504 will simply halt the inputting onthe test input buses 126 to the core pair that has been tested andcontinue the inputting on the test input buses 126 to the core pairstill being tested. At the end of testing, and if a fail flag signal hasbeen received by the tester 504 on fail flag input 420, the tester willscan out the fail flags 140 from all the tester architectures 100 todetermine which core or cores failed the test.

FIG. 6 illustrates an example whereby different cores 606-612 areembedded within an SOC 602. Each different core includes testarchitecture 100. Since the cores are different, the test input buses126 of their test architectures 100 must be connected to separate testinput buses 614-620 from tester 604. During test, the tester test inputbuses 614-620 input separate test pattern sets (i.e. separate stimulus,mask, expected, control, and enable signals) to the test input buses 126of the test architectures 100 of cores 606-612, respectively.

As in the examples of FIGS. 4 and 5, the fail flag outputs 124 of thetest architectures 100 of cores 606-612 are wire-ORed for input totester 504 via fail flag input 420. Also, as with the examples in FIGS.4 and 5, the scan inputs 121 and scan outputs 122 of the testarchitectures 100 of cores 606-612 form a serial path between thetester's scan input 414 and scan output 416. The testing of each coreoccurs as described in regard to the testing cores in FIGS. 4 and 5.

Since the cores are different from one another, one or more cores maycomplete their test before the other cores completes their test. Again,if this happens, tester 604 will simply halt the inputting on the testinput bus 126 to the core(s) that have been tested and continue theinputting on the test input bus 126 to the core(s) still being tested.At the end of testing, and if a fail flag signal has been received bythe tester 604 on fail flag input 420, the tester will scan out the failflags 140 from all the tester architectures 100 to determine which coreor cores failed the test.

It is important to note that in the above descriptions of FIGS. 4-6 thetester 404-604 does not receive scan output from the SOC 402-602 duringtest, since the circuit test architecture 100 does the testing of thelow power scan path 104 output response 128 against expected and maskdata input on test input bus 126 using compare circuitry 106. Thus, withthe exception of the tester having to input the fail flag 420 and scanout 416 signals, the tester is simplified and cost reduced to being an“output only” device to SOC 402-602.

FIG. 7 illustrates an alternative method of designing a low power scanpath 700. Low power scan path 700 is identical to the low power scanpath 104 of FIG. 2 with the one exception that the multiplexers 208-212and gating circuitry 214-218 are controlled by select decode logic 702rather than by select controller 220 of FIG. 2. Also, a select bus 704has been included for inputting select codes to select decode logic 702.The select decode logic 702 is combinational in operation, i.e. a simpledecode circuit. The select decode logic 702 receives input from selectbus 704 and from enable 120. When enabled by the enable input 120, theselect decode logic 702 inputs select codes from the select bus anddecodes the select codes into appropriate multiplexer 208-212 controloutput 228-232 settings and gating circuitry 214-218 control output222-226 settings.

During scan test operations when the enable input 120 is set to enablethe low power mode of low power scan path 700, each parallel scan pathsection A, B, and C is accessed separately by select codes input toselect decode logic 702 such that only portions of the overall stimulusdata signals on bus 110 to combinational logic 102 change at a time. Asmentioned in regard to the low power scan path 104 of FIG. 2, thismethod of scanning reduces power consumption in the combinational logicand in the clock trees feeding the scan paths sections A, B, and C.

During scan test operations when the enable input 120 is not set toenable the low power mode of low power scan path 700, each parallel scanpath section A, B, and C is connected in series together betweenstimulus bus 116 and response bus 128, via multiplexers 208, 210, and212, such that all sections are accessed simultaneously during scan testoperations. This mode of operation duplicates the operation of theconventional parallel scan path 200, and, in doing so, does not providethe low power mode of operation described above. To achieve this non-lowpower mode of operation, the select decode logic 702 is designed suchthat when the enable signal 120 is not set for low power mode, theenable inputs 222-226 to gating circuitry 214-218 are set to allow allthe scan path sections 202-206 to receive scan control from bus 130.Also, the inputs to multiplexers 208-212 are all set to concatenate thescan paths sections 202-206 together between stimulus bus 116 andresponse bus 128.

The reasons for selectively providing both a low power and non-low powermode using the enable signal 120 has been previously described in regardto the low power scan path of FIG. 2.

FIG. 8 illustrates one preferred mode of operation of select decoderlogic 702, when it is enabled, via enable input 120, for the low powermode of operation of scan path 104. The operation is depicted as asequence of actions that take place in response to select code inputs802-808 on select bus 704. At the beginning of each scan cycle, in box802, a shift A select code is input to cause select decoder logic 702 tooutput control to enable operation of or shifting of data in shiftsection A between stimulus bus input 116 and response bus output 128.Next, in box 804, a shift B select code is input to cause the selectdecoder logic 702 to enable operation of or shifting of data in shiftsection B between stimulus bus input 116 and response bus output 128.Next, in box 806, a shift C select code is input to cause the selectdecoder logic 702 to enable operation of or shifting of data in shiftsection C between stimulus bus input 116 and response bus output 128.Next, in box 808, a capture A, B, C select code is input to cause theselect decoder logic 702 to enable operation of all sections A, B, and Cto capture data from combinational logic 102 via connection path 110.

The above described select code input sequence is repeated until testingof combinational logic 102 is complete. The select decoder logic 702 iscapable of responding to the sequence of select code inputs of boxes802-808 such that the test time of the low power scan path 700 is thesame as the conventional non-low power scan path 200.

With the exception of providing selective low power and non-low powermodes of operation, the low power scan path structure and operation asdescribed in FIGS. 7 and 8 above is similar to the structure andoperation of a low power scan path described in pending US patentapplication publication US 2002/0104050 A1, which is incorporated hereinby reference.

FIG. 9 illustrates the core circuit test architecture 900 of FIG. 1 whenthe low power scan path 700 is substituted for the low power scan path104. With the exception of using low power scan path 700 instead of lowpower scan path 104 and the addition of select bus 704 for inputting theselect codes to low power scan path 700, the structure and operation ofthe circuit test architecture 900 is identical to that of testarchitecture 100. The test input bus 902 of circuit test architecture900 differs from test input bus 126 only in that the select bus 704 isincluded as additional test input signals.

One advantage circuit test architecture 900 has over circuit testarchitecture 100 is that the select code inputs to the select decoderlogic 702 can be altered to allow for changing the shifting order ofsections A, B, and C. For example, if it is desired to reverse theshifting order of sections A, B, and C shown in FIG. 8 to say a shiftingorder where the sections are shifted in reverse, i.e. C, B, then A, allthat is necessary is to reverse the select code input sequence on selectbus 704 to the select decoder logic 702. Indeed, any desired shiftingorder of sections A, B, and C can be achieved simply by altering theselect code input sequence. This flexibility is not provided in the lowpower scan path 104 since the shifting order is fixed by the design ofthe select controller 220.

One disadvantage circuit test architecture 900 has that circuit testarchitecture does not have is that it requires additional inputs fromthe tester to supply the select code inputs to select decoder logic 702.Thus when testing must be done by a tester with a minimum of outputsignals, or if it is desired to minimize the test interface signalsbetween the tester and SOC, circuit test architecture 100 may bepreferred over circuit test architecture 900.

FIG. 10 is provided to indicate that circuit test architectures 900 canbe used in SOC 1002 to test identical cores 1006-1012 as circuit testarchitectures 100 were used in SOC 402 to test identical cores 406-412in FIG. 4. The only difference being that a tester 1004 is provided witha test input bus 1014 for inputting to the test input buses 902 ofcircuits 900.

FIG. 11 is provided to indicate that circuit test architectures 900 canbe used in SOC 1102 to test identical core pair 1106 and 1108 andidentical core pair 1110 and 1112 as circuit test architectures 100 wereused in SOC 502 to test identical core pair 506 and 508 and identicalcore pair 510 and 512 in FIG. 5. The only difference being that a tester1104 is provided with a first test input bus 1114 for inputting to thetest input buses 902 of core pair 1106 and 1108, and a second test inputbus 1116 for inputting to the test input buses 902 of core pair 1110 and1112.

FIG. 12 is provided to indicate that circuit test architectures 900 canbe used in SOC 1202 to test different type cores 1206, 1208, 1210, and1212 as circuit test architectures 100 was used in SOC 602 to testdifferent cores 606, 608, 610, and 612 in FIG. 6. The only differencebeing that a tester 1204 is provided with separate test buses 1214,1216, 1218, and 1220 for individually inputting to the test input buses902 of cores 1206, 1208, 1210, and 1212, respectively.

FIG. 13 illustrates how the present disclosure is advantageously used toparallel test multiple die 1306-1312 on a wafer 1302, each diecontaining either FIG. 4 cores 406-412 with test architectures 100 orFIG. 10 cores 1006-1012 with test architectures 900. The test is thesame as previously described for the single SOC of FIGS. 4 and 10. Theonly difference is that the tester 1302 has a test input bus 1314capable of driving the test input buses 1316 of the multiple die1306-1312. If the die use test architecture 100 of FIG. 4, the testinput bus 1316 will be test input bus 126 and the tester test input bus1314 will be test input bus 418. If the die use test architecture 900 ofFIG. 10, the test input bus 1316 will be test input bus 902 and thetester test input bus 1314 will be test input bus 1014.

FIG. 14 illustrates how the present disclosure is advantageously used toparallel test multiple die 1406-1412 on a wafer 1402, each diecontaining either FIG. 5 core pairs 506,508 and 510,512 with testarchitectures 100 or FIG. 11 core pairs 1106,1108 and 1110,1112 withtest architectures 900. The test is the same as previously described forthe single SOC of FIGS. 5 and 11. The only difference is that the tester1402 has test input buses 1414 and 1416 capable of driving the testinput buses 1418 and 1420, respectively, of the multiple die 1406-1412.If the die use test architecture 100 of FIG. 5, the test input buses1418 and 1420 will be test input buses 126 and the tester test inputbuses 1414 and 1416 will be test input buses 508 and 506. If the die usetest architecture 900 of FIG. 11, the test input buses 1418 and 1420will be test input buses 902 and the tester test input buses 1414 and1416 will be test input buses 1114 and 1116.

FIG. 15 illustrates how the present disclosure is advantageously used toparallel test multiple die 1506-1512 on a wafer 1502, each diecontaining either FIG. 6 cores 606-612 with test architectures 100 orFIG. 12 cores 1206-1212 with test architectures 900. The test is thesame as previously described for the single SOC of FIGS. 6 and 12. Theonly difference is that the tester 1502 has test input buses 1514-1520capable of driving the test input buses 1522-1528, respectively, of themultiple die 1506-1512. If the die use test architecture 100 of FIG. 6,the test input buses 1522-1528 will be test input buses 126 and thetester test input buses 1514-1520 will be test input buses 614-620. Ifthe die use test architecture 900 of FIG. 12, the test input buses1522-1528 will be test input buses 902 and the tester test input buses1514-1520 will be test input buses 1214-1220.

FIG. 16 illustrates how the present disclosure is advantageously used toparallel test multiple packaged ICs 1606-1612 on a multiple IC testfixture 1602. For simplification, it is assumed that the ICs 1606-1612in FIG. 16 are simply the die 1306-1312 of FIG. 13 that have passed thewafer level test of FIG. 13, been singulated, and assembled into thepackaged ICs 1606-1612 of FIG. 16. The test applied to the die 1306-1312on wafer 1302 in FIG. 13 is simply repeated to test the packaged ICs1606-1612 on test fixture 1602 of FIG. 16.

FIG. 17 illustrates how the present disclosure is advantageously used toparallel test multiple packaged ICs 1706-1712 on a multiple IC testfixture 1702. Again for simplification, it is assumed that the ICs1706-1712 in FIG. 17 are simply the die 1406-1412 of FIG. 14 that havepassed the wafer level test of FIG. 14, been singulated, and assembledinto the packaged ICs 1706-1712 of FIG. 17. Again, the test applied tothe die 1406-1412 on wafer 1402 in FIG. 14 is simply repeated to testthe packaged ICs 1706-1712 on test fixture 1702 of FIG. 17.

FIG. 18 illustrates how the present disclosure is advantageously used toparallel test multiple packaged ICs 1806-1812 on a multiple IC testfixture 1802. Again for simplification, it is assumed that the ICs1806-1812 in FIG. 18 are simply the die 1506-1512 of FIG. 15 that havepassed the wafer level test of FIG. 15, been singulated, and assembledinto the packaged ICs 1806-1812 of FIG. 17. Again, the test applied tothe die 1506-1512 on wafer 1502 in FIG. 15 is simply repeated to testthe packaged ICs 1806-1812 on test fixture 1802 of FIG. 18.

It is important to note in FIGS. 13-18 that the present disclosureallows for; (1) the same low cost tester 1304 to test both the die1306-1312 of FIG. 13 and the ICs 1606-1612 of FIG. 16, (2) the same lowcost tester 1404 to test both the die 1406-1412 of FIG. 14 and the ICs1706-1712 of FIG. 17, and (3) the same low cost tester 1504 to test boththe die 1506-1512 of FIG. 15 and the ICs 1806-1812 of FIG. 18.

It is also important to note that the ability of the present disclosureto test the multiple die 1306-1312 in FIG. 13 and multiple ICs 1606-1612in FIG. 16 in the time it would take to test a single die or IC,significantly reduces test time and therefore the cost to manufacturedie and ICs. The number of die or ICs that may be tested at the sametime is limited only by the capability of the tester to drive the testinput buses of the multiple die or ICs, the ability to provide physicalcontact between the tester and multiple die or ICs, and, especially inregard to die on wafer testing, the ability to manage power consumptionand heat generation during test. The low power test architectures 100and 900 of the present disclosure contributes largely to the managingthe power consumption and heat generation when compared to usingconventional test architectures that do not address lowering the powerand heat during test.

FIG. 19 is provided to illustrate how the present disclosure may befurther exploited to enable multiple wafers 1302 on a multiple wafertest fixture 1902 to be tested in parallel. The same tester 1304 used totest multiple die 1306-1312 on wafer 1302 in FIG. 13 is reused to testthe multiple die 1306-1312 of the multiple wafers 1302 in FIG. 19.During test each wafer 1302 receives input from the tester's test inputbus 1314 and outputs their fail flags 124 to the tester's wire-ORed failflag input 420. At the end of test, the tester scan through each wafer1302 using its scan input 414 and scan output 416 to retrieve fail flag140 values from each die 1306-1312. Multiple ones of the other wafers1402 and 1502 of FIGS. 14 and 15 could be similarly tested in parallelby similarly placing them into a multiple wafer test fixture adapted forthem and applying tests using testers 1404 and 1504, respectively.

Although the present disclosure has been described in detail, it shouldbe understood that various changes, substitutions and alterations can bemade herein without departing from the spirit and scope of thedisclosure as defined by the appended claims.

What is claimed is:
 1. An integrated circuit comprising: A. functionalcircuitry; B. serial scan circuitry having stimulus leads and responseleads connected to the functional circuitry, the serial scan circuitryhaving a serial input and a serial output; C. a mask input lead; D. anexpected data input lead; E. compare circuitry including a responseinput connected to the serial output of the serial scan circuitry, amask input connected to the mask input lead, and an expected inputconnected to the expected data input lead, the compare circuitryincluding a scan cell separate from the serial scan circuitry, the scancell including a serial scan input, a control input, a serial scanoutput, and a compare input coupled to the response input, the maskinput, and the expected input.
 2. The integrated circuit of claim 1 inwhich the serial scan circuitry includes separate scan paths thatselectively are operated in series between the serial input and serialoutput and are operated separately between the serial input and serialoutput.